<p>APIs are a form of plugins that are tightly associated with a module. Instead of a module providing any number of plugins, each module provides only one file for an API and this file can contain hooks that the module should invoke.</p>

<p>Modules support this API by implementing hook_ctools_plugin_api($module, $api). If they support the API, they return a packet of data:</p>

<pre>
function mymodule_ctools_plugin_api($module, $api) {
  if ($module == 'some module' && $api = 'some api') {
    return array(
      'version' => The minimum API version this system supports. If this API version is incompatible then the .inc file will not be loaded.
      'path' => Where to find the file. Optional; if not specified it will be the module's directory.
      'file' => an alternative version of the filename. If not specified it will be $module.$api.inc
    );
  }
}
</pre>

<p>This implementation must be in the .module file.</p>

<p>Modules utilizing this can invole ctools_plugin_api_include() in order to ensure all modules that support the API will have their files loaded as necessary. It's usually easiest to create a small helper function like this:</p>

<pre>
define('MYMODULE_MINIMUM_VERSION', 1);
define('MYMODULE_VERSION', 1);

function mymodule_include_api() {
  ctools_include('plugins');
  return ctools_plugin_api_include('mymodule', 'myapi', MYMODULE_MINIMUM_VERSION, MYMODULE_VERSION);
}
</pre>

<p>Using a define will ensure your use of version numbers is consistent and easy to update when you make API changes. You can then use the usual module_invoke type commands:</p>

<pre>
mymodule_include_api();
module_invoke('myhook', $data);
</pre>

<p>If you need to pass references, this construct is standard:</p>

<pre>
foreach (mymodule_include_api() as $module => $info) {
  $function = $module . '_hookname';
  // Just because they implement the API and include a file does not guarantee they implemented
  // a hook function!
  if (!function_exists($function)) {
    continue;
  }

  // Typically array_merge() is used below if data is returned.
  $result = $function($data1, $data2, $data3);
}
</pre>

<p>TODO: There needs to be a way to check API version without including anything, as a module may simply
provide normal plugins and versioning could still matter.</p>
